1. Field of the Invention
The present invention relates to a multi-beam semiconductor laser with a plurality of laser emitting elements arranged one- or two-dimensionally. More particularly, the present invention relates to a multi-beam semiconductor laser constructed to freely change the pitch between beams.
2. Description of the Related Art
FIG. 1 shows a basic structure of a conventional semiconductor laser. Referring to FIG. 1, the semiconductor laser is formed from a stack of an n-cladding layer 100, an active layer 101, a p-cladding layer 102, insulating layers 103 and 105 respectively located on the sides of a mesa portion of the p-cladding layer 102, an n-electrode layer 106 formed on the bottom of the n-cladding layer 100, and a p-electrode layer 107 formed on the mesa portion of the p-cladding layer 102. While the n- and p-cladding layers 100 and 102 act to confine carriers or light, the intermediate active layer 101 generates laser energy.
The above-mentioned structure of the semiconductor laser allows the current from the electrode layer 107 to flow into only a striped region of the p-cladding layer 102 between the insulating layers 103 and 105, so that the active layer 101 adjacent to the striped region is activated to emit light from only end surfaces in the stack. Example of this type of structure has been disclosed in Japanese Patent Laid-open Publication Nos. hei 05-218592, hei 10-144991, and 2000-133879.
FIG. 2 shows an example of an edge-emitting multi-beam semiconductor laser with a structure improved over the conventional structures discussed in the above-cited references. Referring to FIG. 2, the edge-emitting multi-beam semiconductor laser includes an n-electrode layer 110, an n-cladding layer 111, an active layer 112, a p-cladding layer 113, an insulating layer 115, and an electrode layer 116 formed in a stack. The semiconductor layer further includes an insulating barrier layer 117 dividing the electrode layer 116, the insulating layer 115, the p-cladding layer 113, the active layer 112, and the n-cladding layer 111 into two regions in the depth direction. The presence of striped portions of the p-cladding layers 113 located in gaps of the two insulating layers 115 separated by the barrier layer 117 allow currents to be injected from the electrode layers 116 into the active layer 112 so that active regions within the active layer 112 emit multiple laser beams.
FIG. 3 shows another example of an edge-emitting multi-beam semiconductor laser. Referring to FIG. 3, like the semiconductor laser of FIG. 10, the edge-emitting multi-beam semiconductor laser includes a stack of an n-electrode layer 120, an n-cladding layer 121, an active layer 122, a p-cladding layer 123, an insulating layer 125, and a p-electrode layer 126. The semiconductor laser further includes a separation groove 127 partitioning the electrode layer 126, the insulating layer 125, the p-cladding layer 123, the active layer 122, and the n-cladding layer 121 into two parts in the depth direction. This structure allows current to be injected from striped portions of the p-cladding layer 123 located in gaps of the two insulating layers 125 separated by the separation groove 127 into the active layer 122 so that active regions within the active layer 122 emit multiple laser beams.
Despite the difference between the barrier layer 117 and the separation groove 127, the semiconductor lasers shown in FIGS. 2 and 3 are a multi-beam semiconductor laser M basically having mirrors on longitudinal end surfaces 130 of a stack in FIG. 4. When laser oscillation lasing takes place at a level higher than a threshold, a laser beam is emitted from the end surfaces 130 of the stack.
FIG. 4 shows a state in which emitted laser beams B1 and B2 are projected. A distance between the centers of the elliptical projected beams B1 and B2 is defined as the pitch P1 between the beams. There has been a need to modify the pitch P1 or reduce it as much as possible when the multi-beam semiconductor laser M is combined with a laser product.
However, the conventional edge-emitting semiconductor lasers shown in FIGS. 2 and 3 have a problem in that an epitaxial growth process must be changed and the electrical characteristics, optical output power, and reliability characteristics of a laser must be reset each time the pitch P1 between beams changes. Another problem is that it is physically impossible to reduce the beam pitch P1 to less than the aperture of a waveguide, or in other words, the width of an active region within an active layer. Also, there are performance problems caused by thermal effects when reducing the beam pitch P1. For example, reducing the beam pitch P1 causes the output of an adjacent laser to change due to the thermal effect when a laser switches from “on” to “off” and vice versa. As a result, a laser diode Ld2 generates an output wave form as crosstalk that is affected by output wave form of another laser diode Ld1 as shown in FIG. 5, thereby, causing a possible power down of the dominant laser diode Ld1.
Accordingly, there is a need for a multi-beam semiconductor laser capable of changing the pitch of the laser beams without affecting the adjacent lasers.